TY - THES

T1 - Fatigue strength assessment of additively manufactured 17-4PH structures considering surface layer effects and residual stresses

AU - Entfellner, Peter

N1 - embargoed until 18-02-2026

PY - 2021

Y1 - 2021

N2 - The objective of this research work is to make a scientic contribution to the evaluation of the fatigue strength of 17-4 PH steel structures manufactured by the selective laser melting process. Four series of samples were investigated, which dier in the post-heat treatment as well as in the type of contour layer used during the printing process. Since residual stresses have a great in uence on the fatigue strength of additively manufactured components, they are determined experimentally in this work utilizing X-ray diraction. An attempt is also made to evaluate the residual stresses numerically using the program SimufactAdditive and to compare them with the experimentally determined results. Furthermore, the in uence of surface roughness on fatigue strength is also considered. The three-dimensional surface topography scanned by a digital optical microscope, form the basis of determining representative areal roughness parameters. The roughness in uence is considered by a local and a global approach with the factors average notch valley depth Sv and the notch radii ρ. The approach divides the specimen surface into sub-areas and considers local roughness parameters, resulting in a direct fatigue strength assessment of the individual areas. A global approach averages the maxima of the surface parameters over the entire specimen surface and also includes the experimentally or numerically determined residual stresses in the fatigue strength prediction. The numerical simulation of the build-up process of the specimens and the resulting residual stresses showed an average deviation of 29 % from the experimentally evaluated residual stresses for three series and a deviation of more than a factor of three for one series. From this, it can be deduced that more precise parameters about the real manufacturing process are necessary to achieve more precise results. When considering the fatigue strength estimation methods, the local concept can shows the lowest average deviation of -8 % from the experimentally determined strengths. The global concept has an average deviation of -16 % with the experimentally determined residual stresses and a value of -18 % with the numerically determined residual stresses. Thus, both methodologies lead to conservative fatigue strength estimation for additively manufactured structures exhibiting as-built surface layer.

AB - The objective of this research work is to make a scientic contribution to the evaluation of the fatigue strength of 17-4 PH steel structures manufactured by the selective laser melting process. Four series of samples were investigated, which dier in the post-heat treatment as well as in the type of contour layer used during the printing process. Since residual stresses have a great in uence on the fatigue strength of additively manufactured components, they are determined experimentally in this work utilizing X-ray diraction. An attempt is also made to evaluate the residual stresses numerically using the program SimufactAdditive and to compare them with the experimentally determined results. Furthermore, the in uence of surface roughness on fatigue strength is also considered. The three-dimensional surface topography scanned by a digital optical microscope, form the basis of determining representative areal roughness parameters. The roughness in uence is considered by a local and a global approach with the factors average notch valley depth Sv and the notch radii ρ. The approach divides the specimen surface into sub-areas and considers local roughness parameters, resulting in a direct fatigue strength assessment of the individual areas. A global approach averages the maxima of the surface parameters over the entire specimen surface and also includes the experimentally or numerically determined residual stresses in the fatigue strength prediction. The numerical simulation of the build-up process of the specimens and the resulting residual stresses showed an average deviation of 29 % from the experimentally evaluated residual stresses for three series and a deviation of more than a factor of three for one series. From this, it can be deduced that more precise parameters about the real manufacturing process are necessary to achieve more precise results. When considering the fatigue strength estimation methods, the local concept can shows the lowest average deviation of -8 % from the experimentally determined strengths. The global concept has an average deviation of -16 % with the experimentally determined residual stresses and a value of -18 % with the numerically determined residual stresses. Thus, both methodologies lead to conservative fatigue strength estimation for additively manufactured structures exhibiting as-built surface layer.

KW - SLM

KW - 17-4PH

KW - SLM

KW - 17-4PH

M3 - Master's Thesis

ER -